Light-receiving element and optical integrated circuit

ABSTRACT

A light-receiving element ( 10 ) according to the present invention includes a semiconductor layer ( 100 ) including a p-type semiconductor region ( 101 ), an n-type semiconductor region ( 102 ), and a multiplication region( 103 ), and a p-type light absorption layer ( 104 ) formed on the multiplication region. The p-type semiconductor region and the n-type semiconductor region are formed to sandwich the multiplication region in a planar direction of the semiconductor layer. This allows an easy implementation of a light-receiving element that serves as an avalanche photodiode by a monolithic manufacturing process.

TECHNICAL FIELD

The present invention relates to a light-receiving element that convertsan optical signal to an electrical signal, and an optical integratedcircuit including the light-receiving element, and relates to, forexample, a light-receiving element that makes full use of a function asan avalanche photodiode.

BACKGROUND ART

A general optical receiver in optical communication is generally formedby a light-receiving element and a transimpedance amplifier (to be alsoreferred to as a “TIA” hereinafter) that amplifies a photocurrentgenerated by the light-receiving element.

Examples of the light-receiving element used in the optical receiver area photodiode (to be also referred to as a “PD” hereinafter) and anavalanche photodiode (to be also referred to as an “APD” hereinafter).

The PD has a function of converting incident light to a current. Theupper limit of the photoelectric conversion efficiency of the PD is 100%as quantum efficiency. As the PD, a uni-traveling carrier photodiode(UTC-PD) and the like are known in addition to a general element made ofa III-V compound semiconductor such as InGaAs (see, for example,non-patent literature 1).

On the other hand, the APD is a light-receiving element having afunction of making photoelectrons generated in the element hit a latticeby accelerating them under a high electric field and thus ionizing thephotoelectrons, thereby amplifying the carriers. The APD can output aplurality of carriers in correspondence with one photon, and thus obtaina sensitivity higher than 100% as the quantum conversion efficiency. Forthis reason, the APD is widely used for a high-sensitivity opticalreceiver (see, for example, non-patent literature 2).

In recent years, for the purpose of downsizing an optical receiver andreducing the cost of the optical receiver, research and development ofmonolithic integration of integrating an optical waveguide including anoptical multiplexer and an optical demultiplexer, a light-receivingelement, a TIA, and the like into a single IC chip are attractingattention. Particularly, “silicon photonics” of sharing a silicon(Si)-based IC and a manufacturing process and manufacturing an opticalactive element such as a light-receiving element has been extensivelyresearched and developed (see, for example, non-patent literature 3).

By applying a silicon photonics technique to an optical receiver, theintegration and formation of a light-receiving element and a CMOS(Complementary Metal Oxide Semiconductor) circuit on silicon (Si) or SOI(Silicon On Insulator) together become possible. Thus, it is possible toreduce the cost in terms of the mass productivity of the opticalreceiver, the stability of the manufacturing process, packaging, andinspection.

In recent research and development of the optical receiver by siliconphotonics, a method of performing, on a silicon substrate, crystalgrowth of germanium (Ge) having sensitivity in a 1.3-μm band and havinga relatively small difference in lattice constant with respect tosilicon, a method of growing, on an InP substrate, InGaAs functioning asa light absorption layer and then transferring InGaAs onto an Sisubstrate by, for example, bonding, or the like is used. For the purposeof improving the sensitivity of the light-receiving element, an APDincluding a multiplication layer made of silicon (Si) has also beenresearched and developed.

As an APD by silicon photonics, a “vertical incident type” APD having astructure for stacking a light absorption layer on an Si or SOIsubstrate and performing voltage application and light injection in adirection parallel to the stacking direction, or a “normal incidenttype” APD is known.

In addition to the above APDs, a waveguide type APD by silicon photonicsis known. The waveguide type APD has the feature that an opticalwaveguide and a light-receiving unit can be integrated and it isunnecessary to use a spatial optical system at the time ofimplementation. As a waveguide type APD, for example, non-patentliterature 4 discloses an APD whose degree of integration of devices isimproved by accelerating light absorption by forming a waveguide in anSi substrate and injecting a fault into the waveguide, and composing, byonly Si, a material for applying an electric field in the waveguide andmultiplying it.

Non-patent literature 5 discloses a waveguide type APD in which acontact layer is provided in each waveguide having functions as a Gelight absorption layer and an Si multiplication layer and evanescentcoupling is used for optical coupling between the Ge light absorptionlayer and the waveguide.

RELATED ART LITERATURE Non-Patent Literature

Non-Patent Literature 1: T. Ishibashi et al., “Uni-Traveling-CarrierPhotodiodes”, in proceedings of Ultrafast Electronics andOptoelectronics, Vol. 13, Optical Society of America, 1997.

Non-Patent Literature 2: J. C. Campbell, “Recent Advances inTelecommunications Avalanche Photodiodes”, IEEE JOURNAL OF LIGHTWAVETECHNOLOGY, Vol. 25, No. 1, January 2007.

Non-Patent Literature 3: B. Jalali et al., “Silicon Photonics”, IEEEJOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 24, No. 12, December 2006.

Non-Patent Literature 4: J. J. Ackert et al., “10 Gbps siliconwaveguide-integrated infrared avalanche photodiode”, OPTICS EXPRESS,Vol. 21, 19530, August, 2013.

Non-Patent Literature 5: N. Duan et al., “High SpeedWaveguide-Integrated Ge/Si Avalanche Photodetector”, in proceedings ofOFC/NFOEC Technical Digest, OSA, 2013.

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, the above-described conventional APDs have the followingproblems.

For example, the vertical incident type APD matches a CMOS process butthe layer arrangement is complicated. It is thus necessary to add anenormous number of man-hours, thereby complicating the manufacturingprocess. The waveguide type APD has lower sensitivity due to the problemof optical coupling efficiency or the optical absorption coefficient ofthe light absorption layer, and the high gain performance and bandperformance cannot be simultaneously implemented because of thenonuniformity of the electric field being applied to the Simultiplication layer, thereby limiting the increase in operating speed.

That is, when a vertical incident structure is applied to as an APD, itis possible to improve the sensitivity and to increase the speed, but aproblem of the more complicated manufacturing process of the APD becomesevident. When a waveguide structure is applied to as an APD, it ispossible to relatively easily implement high-density integration with aCMOS IC by manufacturing the APD in accordance with the manufacturingprocess of the CMOS IC, but a problem of a functionality as an APD, thatis, insufficiencies in the high sensitivity performance and high speedperformance becomes evident.

The present invention has been made in consideration of the aboveproblems, and an object of the present invention is to easily implementa light-receiving element that serves as an APD by a monolithicmanufacturing process.

Means of Solution to the Problem

According to the present invention, there is provided a light-receivingelement that comprises a semiconductor layer including a p-typesemiconductor region, an n-type semiconductor region, and amultiplication region, and a p-type light absorption layer formed on themultiplication region, wherein the p-type semiconductor region and then-type semiconductor region are formed to sandwich the multiplicationregion in a planar direction of the semiconductor layer.

Effect of the Invention

According to the present invention, it is possible to easily implement alight-receiving element that serves as an APD by a monolithicmanufacturing process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view schematically showing a light-receivingelement according to the first embodiment;

FIG. 2 is a view for explaining the operation principle of thelight-receiving element according to the first embodiment;

FIG. 3 is a sectional view schematically showing a light-receivingelement according to the second embodiment;

FIG. 4 is a band diagram showing a change in band gap energy in thestacking direction of the respective layers of the light-receivingelement according to the second embodiment;

FIG. 5 is a sectional view schematically showing a light-receivingelement according to the third embodiment;

FIG. 6 is a band diagram showing a change in band gap energy in thestacking direction of the respective layers of the light-receivingelement according to the third embodiment;

FIG. 7 is a sectional view schematically showing a light-receivingelement according to the fourth embodiment;

FIG. 8 is a band diagram showing a change in band gap energy in thestacking direction of the respective layers of the light-receivingelement according to the fourth embodiment;

FIG. 9 is a plan view schematically showing an optical integratedcircuit including a light-receiving element and an optical waveguideaccording to the present invention;

FIG. 10 is a sectional view schematically showing the optical integratedcircuit taken along a line A-A′ in FIG. 9;

FIG. 11 is a sectional view schematically showing the optical integratedcircuit taken along a line B-B′ in FIG. 9; and

FIG. 12 is a sectional view schematically showing a light-receivingelement according to the sixth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Outline of Light-Receiving Element and Optical Integrated Circuit ofPresent Invention

(1) According to the present invention, there is provided alight-receiving element (10-14) that comprises a semiconductor layer(100) including a p-type semiconductor region (101), an n-typesemiconductor region (102), and a multiplication region (103), and ap-type light absorption layer (104; 114; 124; 134) formed on themultiplication region, wherein the p-type semiconductor region and then-type semiconductor region are formed to sandwich the multiplicationregion in a planar direction of the semiconductor layer.

(2) In the light-receiving element (11), the light absorption layer(114) may have a band gap that decreases toward the multiplicationregion.

(3) The light-receiving element (12) may further comprise a barrierlayer (107) that is formed on the light absorption layer, and has aconduction band edge energy higher than that of the light absorptionlayer.

(4) In the light-receiving element (13), the light absorption layer(124) may have a p-type impurity concentration that decreases toward themultiplication region.

(5) In the light-receiving element, the multiplication region mayinclude Si, the light absorption layer may include Ge_(x)Si_(1-x), andthe light absorption layer may have a composition ratio x ofGe_(x)Si_(1-x) that increases toward the multiplication region.

(6) In the light-receiving element (11), the multiplication region mayinclude Si, the light absorption layer (114) may include Ge_(x)Si_(1-x),and the light absorption layer may have a composition ratio x ofGe_(x)Si_(1-x) that decreases toward the multiplication region.

(7) In the light-receiving element (13), the multiplication region mayinclude a III-V compound semiconductor, the light absorption layer (124)may include a III-V compound semiconductor, and a composition ratio ofthe III-V compound semiconductor forming the light absorption layer maydecrease toward the multiplication region.

(8) In the light-receiving element (14), the multiplication region mayinclude Si, and the light absorption layer (134) may include a III-Vcompound semiconductor (InGaAs).

(9) According to the present invention, there is provided an opticalintegrated circuit (20) that comprises a light-receiving element(10-14), a core (140) that is formed on a semiconductor layer andoptically coupled to a light absorption layer, and a clad layer (141)formed on the core.

Note that in the above description, constituent elements in drawingscorresponding to those of the present invention are denoted by referencenumerals in parentheses, as an example.

2. Embodiments

Embodiments of a light-receiving element and an optical integratedcircuit according to the present invention will be described below withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a sectional view schematically showing a light-receivingelement according to the first embodiment.

A light-receiving element 10 shown in FIG. 1 has a structure in which alight absorption layer 104 is stacked on a multiplication region 103formed between a p-type semiconductor region 101 and an n-typesemiconductor region 102 in a semiconductor layer 100.

The light-receiving element 10 functions as an APD of makingphotoelectrons generated in the light absorption layer 104 by lightirradiation and diffused to the multiplication region 103 hit a latticein the multiplication region 103 by accelerating them by a high electricfield based on a reverse bias applied between the p-type semiconductorregion 101 and the n-type semiconductor region 102, and thus ionizingthe photoelectrons, thereby amplifying the carriers. The light-receivingelement 10 will be described in detail below.

As shown in FIG. 1, the light-receiving element 10 includes thesemiconductor layer 100, the light absorption layer 104, electrodelayers 106, and insulating layers 105.

The semiconductor layer 100 includes the p-type semiconductor region101, the n-type semiconductor region 102, and the multiplication region103. For example, a silicon (Si) substrate or an SOI (Silicon OnInsulator) substrate can be exemplified as the semiconductor layer 100.

The p-type semiconductor region 101 and the n-type semiconductor region102 are formed in the semiconductor layer 100. The p-type semiconductorregion 101 is formed by, for example, ion implantation of boron (B) intothe surface Si layer of the Si or SOI substrate serving as thesemiconductor layer 100. The n-type semiconductor region 102 is formedby, for example, ion implantation of arsenic (As) into the surface Silayer of the Si or SOI substrate serving as the semiconductor layer 100.

As shown in FIG. 1, the p-type semiconductor region 101 and the n-typesemiconductor region 102 are formed to sandwich the multiplicationregion 103 in the planar direction (for example, an X direction inFIG. 1) of the semiconductor layer 100.

The multiplication region 103 is a region for amplifying carriers byaccelerating, by the high electric field, photoelectrons moving from thelight absorption layer 104. More specifically, the multiplication region103 is a region of the semiconductor layer 100, where no impurity forforming the p-type semiconductor region 101 or the n-type semiconductorregion 102 is injected. For example, when the semiconductor layer 100 isformed from the Si or SOI substrate, the multiplication region 103 is aregion made of Si.

In a CMOS process, the length in the X direction of the multiplicationregion 103 (the distance in the X direction between the p-typesemiconductor region 101 and the n-type semiconductor region 102) isappropriately settable within a range of, for example, 20 nm to 200 nm.

The light absorption layer 104 is a layer for generating electron-holepairs by light irradiation, and is formed in contact with themultiplication region 103 of the semiconductor layer 100. The lightabsorption layer 104 is made of a p-type semiconductor material. As thep-type semiconductor material, Ge can be exemplified. As an impurity tobe injected into the light absorption layer 104, B can be exemplified.

The electrode layers 106 constitute electrodes for applying voltages tothe p-type semiconductor region 101 and the n-type semiconductor region102, respectively, and are formed on the p-type semiconductor region 101and the n-type semiconductor region 102 in the semiconductor layer 100,respectively. The electrode layers 106 are made of a metal materialcontaining, for example, tungsten (W) or copper (Cu) as a maincomponent.

The insulating layers 105 are used to insulate the light absorptionlayer 104 and the electrode layers 106. The insulating layers 105 aremade of, for example, silicon oxide (for example, SiO₂).

A method of manufacturing the light-receiving element 10 according tothis embodiment will be briefly explained.

First, the p-type semiconductor region 101 and the n-type semiconductorregion 102 are formed in the surface Si layer of the Si or SOI substrateserving as the semiconductor layer 100. More specifically, the p-typesemiconductor region 101 is formed by injecting B into the surface Silayer of the Si or SOI substrate while forming the n-type semiconductorregion 102 by injecting As at a predetermined interval with respect tothe p-type semiconductor region 101 in the planar direction of thesemiconductor layer 100. The impurities are injected by, for example, awell-known ion implantation method or the like.

This forms the p-type semiconductor region 101 and the n-typesemiconductor region 102, and a region where no impurity is dopedbetween the p-type semiconductor region 101 and the n-type semiconductorregion 102 is formed as the multiplication region 103.

Note that as the interval between the p-type semiconductor region 101and the n-type semiconductor region 102 in the planar view, a distanceenough for the region between the p-type semiconductor region 101 andthe n-type semiconductor region 102 to function as the multiplicationregion 103 is ensured.

Next, an insulating film is formed on the semiconductor layer 100. Forexample, a silicon oxide layer (SiO₂) is formed on the semiconductorlayer 100 by a plasma CVD method. An opening is formed in a region of anupper portion of the multiplication region 103 in the silicon oxidelayer. For example, the region of the upper portion of themultiplication region 103 in the silicon oxide layer is selectivelyremoved by a well-known photolithography technique and dry etchingtechnique.

Next, the light absorption layer 104 is formed by selectively growing Geon the multiplication region 103 in the semiconductor layer 100 using,as a selective growth mask, the silicon oxide layer in which the openinghas been formed. For example, it is possible to selectively grow Ge onthe multiplication region 103 in the semiconductor layer 100 bydepositing Ge under a substrate temperature condition of 600° C. by aCVD (Chemical Vapor Deposition) method using GeH₄ as a source gas. Torelax large lattice mismatching between the Si layer and the Ge lightabsorption layer, Ge_(x)Si_(1-x) may be grown between the Si layer andthe Ge light absorption layer. In this selective growth, no Ge isdeposited on the selective growth mask.

Note that the light absorption layer 104 may exhibit p-type conductivityby adding a boron compound (for example, B₂H₆) together with the sourcegas or by injecting B as an impurity by, for example, ion implantationafter selectively growing Ge.

Next, the electrode layers 106 are formed. The silicon oxide layer(insulating film) deposited on the p-type semiconductor region 101 andthe n-type semiconductor region 102 in the semiconductor layer 100 isselectively removed by, for example, a well-known photolithographytechnique and dry etching technique. Then, the electrode layers 106 madeof a metal material containing, for example, tungsten (W) or copper (Cu)are formed on the p-type semiconductor region 101 and the n-typesemiconductor region 102 by, for example, a well-known lift-off method.More specifically, after a resist pattern used to selectively remove thesilicon oxide layer is left and the metal material containing W or Cu isdeposited on the semiconductor layer 100 and resist pattern, the resistpattern is removed. This forms the electrode layers 106 on the p-typesemiconductor region 101 and the n-type semiconductor region 102. Todeposit W, for example, a well-known evaporation method is used. Todeposit Cu, for example, a well-known plating method is used.

The silicon oxide layer (insulating film) between each electrode layer106 and the light absorption layer 104 serves as the insulating layer105.

By executing the above steps, the light-receiving element 10 accordingto the first embodiment can be manufactured.

The principle of the operation of the light-receiving element 10 as anAPD according to the first embodiment will be described next.

FIG. 2 is a view for explaining the operation principle of thelight-receiving element according to the first embodiment.

When a reverse bias voltage is applied between the p-type semiconductorregion 101 and the n-type semiconductor region 102 and the electricfield strength of the multiplication region 103 between the p-typesemiconductor region 101 and the n-type semiconductor region 102 reachesa strength enough for avalanche multiplication, the multiplicationregion 103 exhibits a multiplication function.

On the other hand, when light is injected into the light absorptionlayer 104, electron-hole pairs are generated in the light absorptionlayer 104, as shown in FIG. 2. Since the light absorption layer 104 isdoped into p-type conductivity, it is electrically neutral and noelectric field is generated. The generated electron-hole pairs move inthe light absorption layer 104 by diffusion. That is, electrons in thelight absorption layer 104 move to the multiplication region 103 after adiffusion process, and holes in the light absorption layer 104 move tothe multiplication region 103 after a lapse of a dielectric relaxationtime.

When the film thickness of the light absorption layer 104 (the length ofthe light absorption layer 104 in a direction perpendicular to the planeof the semiconductor layer 100) is large, the electrons and holesgenerated in the light absorption layer 104 are coupled again after alapse of a given carrier life time, and cannot reach the multiplicationregion 103. On the other hand, when the film thickness of the lightabsorption layer 104 is made small to be used for the waveguide typelight-receiving element, it is possible to diffuse and move theelectrons generated in the light absorption layer 104 to themultiplication region 103, and move the holes to the multiplicationregion 103 after a lapse of a dielectric relaxation time. Thus, in thesemiconductor layer 100 according to this embodiment, the film thicknessof the light absorption layer 104 is set to a size that allows theelectrons and holes generated in the light absorption layer 104 to bediffused and moved to the multiplication region 103. More specifically,the film thickness of the light absorption layer 104 is set to aboutseveral hundred nm. This can move the electrons and holes generated inthe light absorption layer 104 to the multiplication region 103. Thatis, in the light-receiving element 10 according to this embodiment, theelectron-hole pairs in the light absorption layer 104 exhibit the samebehavior as that of the uni-traveling carrier photodiode (UTC-PD), andthe carrier transport time of the light absorption layer 104 isdominated by the electrons.

The electrons and holes injected into the multiplication region 103repeat avalanche multiplication by the high electric field in themultiplication region 103. This moves the holes to the p-typesemiconductor region 101 and moves the electrons to the n-typesemiconductor region 102, and thus the light-receiving element 10functions as an APD.

As described above, the light-receiving element 10 according to thefirst embodiment has a structure in which the light absorption layer 104is stacked on the multiplication region 103 formed between the p-typesemiconductor region 101 and the n-type semiconductor region 102 in thesemiconductor layer 100. Therefore, no electric field control layer isrequired, and the layer arrangement is simpler than that of theconventional vertical incident type APD or the like.

For example, the conventional vertical incident type APD having thestructure in which a light absorption layer and a multiplication layerare sandwiched between electrodes requires an electric field controllayer to selectively provide a high electric field strength to only themultiplication layer. The electric field provided to the lightabsorption layer needs to be suppressed to the extent that neitheravalanche breakdown nor Zener breakdown occurs in the light absorptionlayer itself while the carriers reach the saturation velocity. Thus,fine doping control corresponding to the materials and film thicknessesof the light absorption layer and the multiplication layer is requiredfor the electric field control layer. To the contrary, thelight-receiving element 10 according to the first embodiment has nostructure in which the light absorption layer 104 is sandwiched betweenelectrodes (the p-type semiconductor region 101 and the n-typesemiconductor region 102), unlike the conventional APD. Therefore, noelectric field control layer is required, and the manufacturing processis simpler than that for the conventional vertical incident type APD orthe like.

As described above, as the manufacturing process of the light-receivingelement 10 according to the first embodiment, a well-known Si/Ge CMOSprocess can be applied.

Therefore, the light-receiving element 10 according to the firstembodiment can be manufactured by a process simpler than that for theconventional vertical incident type APD and integrated with a CMOS IC ata high density.

Furthermore, the light-receiving element 10 according to the firstembodiment has a structure in which a direction (X direction in FIG. 1)in which a pin junction is formed from the p-type semiconductor region101, the multiplication region 103, and the n-type semiconductor region102 in the semiconductor layer 100 is different from the stackingdirection (Y direction in FIG. 1) of the light absorption layer 104 onthe multiplication region 103. Therefore, an electric field when avoltage is applied between the p-type semiconductor region 101 and then-type semiconductor region 102 almost acts on only the multiplicationregion 103. This can uniformly apply the electric field to themultiplication region 103.

Since part of the electric field also acts on the light absorption layer104 depending on the dielectric constant of the material on thesemiconductor layer 100 but the light absorption layer 104 is doped intop-type conductivity, the light absorption layer 104 is maintained to beelectrically neutral for some electric field. Therefore, in thelight-receiving element 10 according to the first embodiment, anelectric field is hardly generated to the light absorption layer 104 inprinciple. It is thus possible to suppress depletion of the lightabsorption layer 104 and a decrease in operating speed of the APD.

Therefore, unlike the conventional waveguide type APD, in thelight-receiving element 10 according to the first embodiment, theelectric filed is uniformly applied to the multiplication region 103 andit is possible to suppress depletion of the light absorption layer 104.It is thus possible to ensure high sensitivity performance and highspeed performance as an APD. For example, it is possible to implement ahigh gain bandwidth product (GBP) of 300 GHz or more by forming thep-type semiconductor region 101 and the n-type semiconductor region 102so that the distance (the width of the multiplication region 103) in theplanar direction between the p-type semiconductor region 101 and then-type semiconductor region 102 falls within a range of about 100 to 500nm.

As described above, according to the light-receiving element 10 of thefirst embodiment, it is possible to easily implement a light-receivingelement that serves as an APD by a monolithic manufacturing process.

In addition, in the light-receiving element 10 according to the firstembodiment, the light absorption layer 104 is doped into p-typeconductivity, as described above. Thus, an electric field is hardlygenerated to the light absorption layer 104. As compared with theconventional APD using an undoped Ge light absorption layer, it ispossible to reduce a dark current caused by a fault occurring along withlattice mismatching with Si, thereby contributing to improvement oflong-term reliability.

Second Embodiment

FIG. 3 is a sectional view schematically showing a light-receivingelement according to the second embodiment.

A light-receiving element 11 shown in FIG. 3 is different from thelight-receiving element 10 according to the first embodiment in that theband gap of a light absorption layer decreases toward a multiplicationregion but the remaining arrangement is the same as that of thelight-receiving element 10. In the following description, the sameconstituent elements as those of the light-receiving element 10according to the first embodiment are denoted by the same referencenumerals and a detailed description thereof will be omitted.

A light absorption layer 114 of the light-receiving element 11 is madeof, for example, p-type GeSn, and is formed so that a band gap decreasestoward a multiplication region 103. For example, the band gap of thelight absorption layer 114 decreases in a direction (Y direction in FIG.3) perpendicular to the plane of a semiconductor layer 100.

FIG. 4 is a band diagram showing a change in band gap energy in thestacking direction of the respective layers of the light-receivingelement according to the second embodiment.

As shown in FIG. 4, the band gap of the light absorption layer 114 ofthe light-receiving element 11 decreases continuously or stepwise towardthe interface with the semiconductor layer forming the multiplicationregion 103. To implement the light absorption layer 114, for example,the light absorption layer 114 is made of Ge_(x)Sin_(1-x), and a Gecomposition ratio x is decreased toward the multiplication region 103 ina direction (Y direction in FIG. 3) perpendicular to the plane of thesemiconductor layer 100. Furthermore, B can be exemplified as animpurity to be doped in the light absorption layer 114.

Note that as a process when manufacturing the light-receiving element 11according to the second embodiment, a well-known Si/Ge CMOS process isapplied, similarly to the light-receiving element 10 according to thefirst embodiment.

The light-receiving element 11 according to the second embodiment canoperate as an APD at higher speed, as follows.

As described above, carrier transport in the light absorption layer 104is limited by the diffusion rate of electrons. A band structure in whicha conduction band edge shifts to the lower energy side toward themultiplication region 103 by decreasing the band gap of the lightabsorption layer 114 continuously (or stepwise) toward themultiplication region 103, like the light-receiving element 11 accordingto the second embodiment, is obtained. Thus, the electrons in the lightabsorption layer 114 sense an electric field in a pseudo manner and notonly diffusion but also given drift effect is obtained. This can furthershorten the carrier traveling time in the light absorption layer 114,and thus a higher-speed operation as an APD can be expected.

Third Embodiment

FIG. 5 is a sectional view schematically showing a light-receivingelement according to the third embodiment.

A light-receiving element 12 shown in FIG. 5 is different from thelight-receiving element 11 according to the second embodiment in that abarrier layer is formed on a light absorption layer but the remainingarrangement is the same as that of the light-receiving element 11. Inthe following description, the same constituent elements as those of thelight-receiving element 11 according to the second embodiment aredenoted by the same reference numerals and a detailed descriptionthereof will be omitted.

As shown in FIG. 5, the light-receiving element 12 further includes abarrier layer 107 that is formed on a light absorption layer 114 and hasconduction band edge energy higher than that of the light absorptionlayer. More specifically, the barrier layer 107 has a conduction bandedge located on the higher energy side with respect to a semiconductorlayer forming the light absorption layer 114 and is formed from asemiconductor layer doped into p-type conductivity.

FIG. 6 is a band diagram showing a change in band gap energy in thestacking direction of the respective layers of the light-receivingelement 12 according to the third embodiment.

As shown in FIG. 6, a semiconductor layer that has a conduction bandedge located on the higher energy side with respect to the semiconductorlayer forming the light absorption layer 114 and is doped into p-typeconductivity is formed as the barrier layer 107 on the surface of thelight absorption layer 114. Similarly to, for example, thelight-receiving element 11 according to the second embodiment, the lightabsorption layer 114 is made of Si_(x)Ge_(1-x). The barrier layer 107 ismade of, for example, Si. As an impurity to be doped in the barrierlayer 107, B can be exemplified.

Note that as a process when manufacturing the light-receiving element 12according to the third embodiment, a well-known Si/Ge CMOS process canbe applied, similarly to the first and second embodiments.

The light-receiving element 12 according to the third embodiment canfurther improve the sensitivity as an APD, as follows.

As described above in the second embodiment, since the band gap of thelight absorption layer 114 decreases continuously or stepwise toward theinterface with the semiconductor layer forming the multiplication region103, carrier transport in the light absorption layer 114 is limited bythe diffusion rate and drift velocity of electrons. At this time, allthe electrons generated by light injection into the light absorptionlayer 114 ideally move to the multiplication region 103. However, theelectrons may be diffused in a direction opposite to the multiplicationregion 103 (a direction opposite to the direction Y in FIG. 5). Since,especially, the light absorption layer 114 is doped into p-typeconductivity, band bending in which the electrons leak to the surfaceside of the light absorption layer 114 (in the direction opposite to theY direction) may occur unless special passivation is performed, therebydecreasing the photoelectric conversion efficiency.

To solve this problem, by forming, as the barrier layer 107, on thesurface of the light absorption layer 114, a semiconductor layer thathas a conduction band edge located on the higher energy side withrespect to the light absorption layer 114 and is doped into p-typeconductivity, like the light-receiving element 12 according to the thirdembodiment, it is possible to suppress electron diffusion to the surfaceside of the light absorption layer 114 (in the direction opposite to theY direction). This can improve the quantum efficiency in the lightabsorption layer 114, thereby improving the sensitivity as an APD.

The offset of the conduction band edge between the light absorptionlayer 114 and the barrier layer 107 is desirably equal to or larger than30 meV. Assuming that the light-receiving element 12 operates at roomtemperature, it is possible to suppress, by thermal energy, theelectrons from crossing the offset of the conduction band edge betweenthe light absorption layer 114 and the barrier layer 107, therebycontributing to further improvement of the sensitivity.

Fourth Embodiment

FIG. 7 is a sectional view schematically showing a light-receivingelement according to the fourth embodiment.

A light-receiving element 13 shown in FIG. 7 is different from thelight-receiving element 12 according to the third embodiment in that theimpurity concentration of a p-type light absorption layer decreasestoward a multiplication region 103 and the semiconductor layer(substrate) of the light-receiving element is made of a III-V compoundsemiconductor but the remaining arrangement is the same as that of thelight-receiving element 12. In the following description, the sameconstituent elements as those of the light-receiving element 12according to the third embodiment are denoted by the same referencenumerals and a detailed description thereof will be omitted.

In the light-receiving element 13, a semiconductor layer 100 is made ofa III-V compound semiconductor. For example, the semiconductor layer 100is an InP substrate, and the multiplication region 103 is a region ofthe InP substrate, where no impurity is doped. A p-type semiconductorregion 101 is formed by, for example, ion implantation of zinc (Zn) asan impurity into the semiconductor layer 100 formed by the InPsubstrate. An n-type semiconductor region 102 is formed by, for example,ion implantation of Si as an impurity into the semiconductor layer 100formed by the InP substrate.

When the semiconductor layer 100 is the InP substrate, the length in theX direction of the multiplication region 103 (the distance in the Xdirection between the p-type semiconductor region 101 and the n-typesemiconductor region 102) is appropriately settable within a range of,for example, 100 nm to 200 nm.

Electrode layers 106 are made of a metal material containing, forexample, titanium (Ti) or gold (Au) as a main component. A barrier layer107 is made of, for example, InAlAs.

A light absorption layer 124 is made of a III-V compound semiconductor.The light absorption layer 124 is formed so that a band gap decreasescontinuously or stepwise toward the multiplication region 103 and thep-type impurity concentration decreases toward the multiplication region103. The light absorption layer 124 will be described in detail belowwith reference to FIG. 8.

FIG. 8 is a band diagram showing a change in band gap energy in thestacking direction of the respective layers of the light-receivingelement according to the fourth embodiment.

For example, the light absorption layer 124 is made of InAlGaAs, and thecomposition ratio of “Al” of InAlGaAs is decreased in a direction (Ydirection) perpendicular to the plane of the semiconductor layer 100.This can decrease the band gap of the light absorption layer 124continuously or stepwise toward the multiplication region 103, as shownin FIG. 8, similarly to the light-receiving element 11 according to thesecond embodiment and the like.

The impurity doping concentration of the light absorption layer 124 isdecreased toward the multiplication region 103. More specifically, theimpurity doping concentration is decreased within a range of, forexample, 10²⁰ to 10¹⁸ cm⁻³ near the interface with the multiplicationregion 103 in the light absorption layer 124. Since this effectivelycauses band bending near the interface between the light absorptionlayer 124 and the multiplication region 103, as shown in FIG. 8, theelectrons in the light absorption layer 124 are accelerated toward themultiplication region 103 and a higher-speed operation as an APD can beexpected.

As an impurity to be doped in the light absorption layer 104, beryllium(Be) or zinc (Zn) can be exemplified.

Note that since part of an electric field between the p-typesemiconductor region 101 and the n-type semiconductor region 102 reachesthe light absorption layer 124, as shown in FIG. 7, when the impuritydoping concentration of the light absorption layer 124 is made low,partial depletion may occur in the light absorption layer 124 andinjection of holes into the multiplication region 103 by dielectricrelaxation may be suppressed. To prevent this, the light absorptionlayer 124 desirably ensures an impurity doping concentration such thatno partial depletion occurs in the light absorption layer 124 whenoperating as an APD.

Fifth Embodiment

FIG. 9 is a plan view schematically showing an optical integratedcircuit including a light-receiving element and an optical waveguideaccording to the present invention.

FIG. 10 is a sectional view schematically showing the optical integratedcircuit taken along a line A-A′ in FIG. 9. FIG. 11 is a sectional viewschematically showing the optical integrated circuit taken along a lineB-B′ in FIG. 9.

An optical integrated circuit 20 shown in FIGS. 9 to 11 is obtained byforming the light-receiving element and the optical waveguide accordingto the present invention on a semiconductor layer (substrate) 100 bymonolithic manufacturing process.

More specifically, the optical integrated circuit 20 includes alight-receiving element 12, a core 140 optically coupled to a lightabsorption layer 114 of the light-receiving element 12, and a clad layer141 formed on the core 140.

Note that the light-receiving element 12 according to the thirdembodiment is shown as an example of the light-receiving element formingthe optical integrated circuit 20 shown in FIGS. 9 to 11. The presentinvention, however, is not limited to this. Any of the light-receivingelements 10, 11, and 13 according to the first, second, and fourthembodiments may be adopted. In the following description, the sameconstituent elements as those of the light-receiving element 12according to the third embodiment are denoted by the same referencenumerals and a detailed description thereof will be omitted.

As shown in FIGS. 9 and 10, the light-receiving element 12 is formed onthe semiconductor layer 100. As an example, the semiconductor layer 100having a multiplication region 103 is an Si or SOI substrate, a p-typesemiconductor region 101 is formed by ion implantation of B into thesemiconductor layer 100, and an n-type semiconductor region 102 isformed by ion implantation of As into the semiconductor layer 100. Thelight absorption layer 114 is made of p-type Si_(x)Ge_(1-x), andelectrode layers 106 are made of a metal material containing W or Cu asa main component.

As shown in FIGS. 9 and 11, the core 140 is formed on the semiconductorlayer 100. The core 140 is made of, for example, SiN. Furthermore, theclad layer 141 is formed on the semiconductor layer 100 to cover thecore 140. The clad layer 141 is made of, for example, SiO₂. The core 140and the clad layer 141 form an optical waveguide. As shown in FIG. 9,the optical waveguide formed from the core 140 and the clad layer 141 isformed to be optically coupled to the light absorption layer 114 of thelight-receiving element 12.

A method of manufacturing the optical integrated circuit 20 will bebriefly explained.

First, the p-type semiconductor region 101, the n-type semiconductorregion 102, and the multiplication region 103 for the light-receivingelement 12 are formed in the semiconductor layer 100, and then the core140 constituting the optical waveguide is formed on the semiconductorlayer 100. For example, the core 140 is formed by depositing an SiNlayer on the semiconductor layer 100, and then patterning the SiN layer.

Next, the clad layer 141 is formed on the core 140. For example, asilicon oxide layer (for example, SiO₂) is formed on the semiconductorlayer 100 to cover the semiconductor layer 100 and the clad layer 141.For example, the silicon oxide layer is formed by a plasma CVD method.Next, an opening is formed in a region of an upper portion of themultiplication region 103 in the silicon oxide layer. For example, theregion of the upper portion of the multiplication region 103 in thesilicon oxide layer is selectively removed by a well-knownphotolithography technique and dry etching technique.

Next, the light absorption layer 114 is formed by selectively growing,for example, Si_(x)Ge_(1-x) on the multiplication region 103 in thesemiconductor layer 100 using, as a selective growth mask, the siliconoxide layer in which the opening has been formed. For example, similarlyto the manufacturing process of the light-receiving element according tothe first embodiment, Si_(x)Ge_(1-x) is selectively grown on themultiplication region 103 in the semiconductor layer 100 by a well-knownCVD method or the like.

Next, the electrode layers 106 are formed. The silicon oxide layer (cladlayer 141) deposited on the p-type semiconductor region 101 and then-type semiconductor region 102 in the semiconductor layer 100 isselectively removed by, for example, a well-known photolithographytechnique and dry etching technique. Then, the electrode layers 106 madeof a metal material containing, for example, W or Cu are formed on thep-type semiconductor region 101 and the n-type semiconductor region 102by, for example, a well-known lift-off method. More specifically, aftera resist pattern used to selectively remove the silicon oxide layer isleft and the metal material containing W or Cu is deposited on thesemiconductor layer 100 and resist pattern, the resist pattern isremoved. This forms the electrode layers 106 on the p-type semiconductorregion 101 and the n-type semiconductor region 102. To deposit W, forexample, a well-known evaporation method is used. To deposit Cu, forexample, a well-known plating method is used.

By performing the above steps, the optical integrated circuit 20 can bemanufactured.

In the optical integrated circuit 20 according to this embodiment, it ispossible to implement high-quality optical coupling between the opticalwaveguide and the light absorption layer of the light-receiving elementwithout departing from a normal CMOS process. For example, in manyconventional waveguide type APDs, the light absorption layer of the APDand an optical waveguide are optically coupled by evanescent coupling.Therefore, the optical coupling efficiency is insufficient and thelight-receiving sensitivity of the APD is low. To the contrary, in theoptical integrated circuit 20 according to this embodiment, it ispossible to optically couple the optical waveguide and the lightabsorption layer by a butt joint since the optical waveguide is formedas part of a step of forming the light-receiving element as an APD (forexample, a step of forming insulating layers). This can improve theoptical coupling efficiency between the optical waveguide and the lightabsorption layer of the light-receiving element in the opticalintegrated circuit 20, and improve the light-receiving sensitivity ofthe light-receiving element as an APD.

Sixth Embodiment

FIG. 12 is a sectional view schematically showing a light-receivingelement according to the sixth embodiment.

A light-receiving element 14 shown in FIG. 12 is different from thelight-receiving element 10 according to the first embodiment in that alight absorption layer is made of a p-type III-V compound semiconductorbut the remaining arrangement is the same as that of the light-receivingelement 10. In the following description, the same constituent elementsas those of the light-receiving element 10 according to the firstembodiment are denoted by the same reference numerals and a detaileddescription thereof will be omitted.

In the light-receiving element 14, a semiconductor layer 100 is an Si orSOI substrate, and a multiplication region 103 is a region of thesurface Si layer of the Si or SOI substrate, where no impurity is doped.A p-type semiconductor region 101 is formed by, for example, ionimplantation of B as an impurity into the surface Si layer of thesemiconductor layer 100 formed by the Si or SOI substrate. An n-typesemiconductor region 102 is formed by, for example, ion implantation ofAs as an impurity into the surface Si layer of the semiconductor layer100 formed by the Si or SOI substrate.

A light absorption layer 134 is made of, for example, a III-V compoundsemiconductor. The light absorption layer 134 is formed on the Si or SOIsubstrate by a so-called “III-V on Si technique” of bonding the III Vcompound semiconductor on the Si or SOI substrate at a wafer level orchip level. More specifically, for example, p-type InGaAs is epitaxiallygrown on a crystal substrate of InP or the like, and InGaAs and thesemiconductor layer 100 (Si or SOI substrate) undergo wafer bonding to abonding surface 400 of the multiplication region 103. As wafer bonding,a well-known surface activation method is used.

Wafer bonding is performed at the initial stage of the manufacturingprocess of the light-receiving element. For example, after forming thep-type semiconductor region 101 and the n-type semiconductor region 102in the Si or SOI substrate as the semiconductor layer 100, InGaAs havingundergone crystal growth is bonded to the semiconductor layer 100. Afterbonding, similarly to the light-receiving element 10 according to thefirst embodiment, respective layers forming the light-receiving elementare formed by a well-known photolithography technique, dry etching, andthe like which are used in the normal CMOS process. This does not impairthe ease of the manufacturing process even in the light-receivingelement according to the sixth embodiment.

As described above, the light-receiving element according to the sixthembodiment can implement a higher-speed and higher-sensitive APD. Forexample, when Si is used for the multiplication region of the APD, thelight-receiving element is advantageous in terms of high-densityintegration and high gain bandwidth product (GBP). In this case, Ge,GeSn, or the like that has a lattice constant relatively close to thatof Si forming the multiplication region is generally used for the lightabsorption layer. However, the APD using Ge or GeSn for the lightabsorption layer is disadvantageous in terms of the electron mobilityand optical absorption coefficient, as compared with the APD using aIII-V compound semiconductor (InGaAs).

To the contrary, since the light-receiving element according to thesixth embodiment uses Si as the multiplication region 103 of thelight-receiving element 14 and uses a III-V compound semiconductor (forexample, InGaAs) as the light absorption layer 134 by wafer bonding ofdissimilar materials, it is possible to further increase the speed andimprove the sensitivity as an APD.

The invention made by the present inventors has been described in detailabove based on the embodiments. However, the present invention is notlimited to them, and various changes can be made without departing fromthe spirit of the invention.

For example, a case has been exemplified in which the barrier layer 107is formed on the surface of the light absorption layer 114 formed sothat the band gap decreases toward the multiplication region 103 in thelight-receiving element 12 according to the third embodiment. However,even in the light-receiving element according to each of otherembodiments, a barrier layer can be formed on the surface of the lightabsorption layer in the same manner.

The second embodiment has exemplified a case in which the lightabsorption layer 114 is made of p-type Ge_(x)Sn_(1-x), and the Gecomposition ratio x is decreased toward the multiplication region 103 inthe direction perpendicular to the plane of the semiconductor layer 100.The present invention, however, is not limited to this. For example, thelight absorption layer 114 may be made of p-type Ge_(x)Sn_(1-x), and theGe composition ratio x may be increased toward the multiplication region103 in the direction perpendicular to the plane of the semiconductorlayer 100. This can further shorten the carrier traveling time in thelight absorption layer, and thus a higher-speed operation as an APD canbe expected, similarly to the light-receiving element 12 according tothe second embodiment.

In the above embodiments, Si/SiGe, InP/InAlGaAs, and Si/InGaAs have beenexemplified as combinations of materials of the multiplication regionand light absorption layer as an APD. The present invention, however, isnot limited to them and other materials may be combined.

The fifth embodiment has exemplified SiN and SiO₂ as a combination ofthe core 140 and the clad layer 141 forming the optical waveguide. Thepresent invention, however, is not limited to this, and other materialsmay be combined to form an optical waveguide.

In the above embodiments, a reflection film or antireflection film maybe formed in each of the end portions of the light absorption layers 104to 124 of the light-receiving elements 10 to 13. In the opticalintegrated circuit 20, a reflection film or antireflection film may beformed in a bonding portion between the light absorption layer 134 andthe optical waveguide (the core 140 and the clad layer 141) or anincident portion on the optical waveguide. An intermediate layer may beformed appropriately between the electrode layers 106 and the p-typesemiconductor region 101 and n-type semiconductor region 102 or betweenthe multiplication region 103 and each of the light absorption layer 104to 124 in terms of a reduction in ohmic resistance, band alignment, orthe like. Note that these additional elements can be implemented byapplying a well-known semiconductor manufacturing process.

Each of the first to fifth embodiments has exemplified a method offorming a light absorption layer by selectively growing it on themultiplication region 103. The present invention, however, is notlimited to this as long as it is possible to form a light absorptionlayer on the multiplication region 103. For example, a light absorptionlayer may be formed on the multiplication region 103 by wafer bonding,chip bonding, or the like.

INDUSTRIAL APPLICABILITY

A light-receiving element and an optical integrated circuit according tothe present invention can be widely used for, for example, an opticalreceiver in optical communication and the like.

EXPLANATION OF THE REFERENCE NUMERALS AND SIGNS

11, 12, 13 . . . light-receiving element, 20 . . . optical integratedcircuit, 100 . . . semiconductor layer, 101 . . . p-type semiconductorregion, 102 . . . n-type semiconductor region, 103 . . . multiplicationregion, 104, 114, 124, 134 . . . light absorption layer, 105 . . .insulating layer, 106 . . . electrode layer, 107 . . . barrier layer,140 . . . core, 141 . . . clad layer, 400 . . . bonding surface

1. A light-receiving element comprising: a semiconductor layer includinga p-type semiconductor region, an n-type semiconductor region, and amultiplication region; and a p-type light absorption layer formed on themultiplication region, wherein the p-type semiconductor region and then-type semiconductor region are formed to sandwich the multiplicationregion in a planar direction of the semiconductor layer, and the lightabsorption layer has a band gap that decreases toward the multiplicationregion.
 2. (canceled)
 3. The light-receiving element according to claim1, further comprising: a barrier layer that is formed on the lightabsorption layer and has a conduction band edge energy higher than thatof the light absorption layer.
 4. The light-receiving element accordingto claim 1, wherein the light absorption layer has a p-type impurityconcentration that decreases toward the multiplication region.
 5. Thelight-receiving element according to claim 1, wherein the multiplicationregion includes Si, the light absorption layer includes Ge_(x)Si_(1-x),and the light absorption layer has a composition ratio x ofGe_(x)Si_(1-x) that increases toward the multiplication region.
 6. Thelight-receiving element according to claim 1, wherein the multiplicationregion includes Si, the light absorption layer includes Ge_(x)Si_(1-x),and the light absorption layer has a composition ratio x ofGe_(x)Si_(1-x) that decreases toward the multiplication region.
 7. Thelight-receiving element according to claim 4, wherein the multiplicationregion includes a III-V compound semiconductor, the light absorptionlayer includes a III-V compound semiconductor, and a composition ratioof the III-V compound semiconductor forming the light absorption layerdecreases toward the multiplication region.
 8. The light-receivingelement according to claim 1, wherein the multiplication region includesSi, and the light absorption layer includes a III-V compoundsemiconductor.
 9. An optical integrated circuit comprising: thelight-receiving element defined in claim 1; a core that is formed on thesemiconductor layer and optically coupled to a light absorption layer;and a clad layer formed on the core.